USB is not a symmetrical protocol, there must be a dedicated 'host' and 'device' at each end of the cable. Most PDAs do not have USB host ports, so they can act only as USB peripherals. Since the OBD2 cable is a USB peripheral, it won't attach most PDAs.
There have been a couple of USB-host-capable PDAs but they are the exception rather than the norm.
There is a new flavor of USB called USB "on the go" which allows devices to act as host or peripheral, but it's not widely use yet (and the current Tactrix cables don't support it).
I think my PDA (Toshiba e750) might be able to act as a host, but I'm not sure if it will recognize the Tactrix cable without a Windows CE driver for the cable... I will give it a shot if I can find the widget that gives the e750 a physical USB host port.
All that said, if the Tactrix guys can be talked into creating a cable with an OBD2 connector on one end and TTL-level serial connectors on the other end, one could build a pretty sweet logging device using a Gumstix computer (
http://www.gumstix.com/). Some of the Gumstix boards have MMC slots to store logs, or it can be connected to a PC using USB. Cooler still if the OBD2 cable can provide power and ground. Might need 5V regulation, but that shouldn't be hard.